Evidence That Soil Aluminum Enforces Site Fidelity of Southern New England Forest Trees Author(s): Seth W. Bigelow and Charles D. Canham Source: Rhodora, 112(949):1-21. 2010. Published By: The New England Botanical Club, Inc. DOI: 10.3119/08-32.1 URL: http://www.bioone.org/doi/full/10.3119/08-32.1 BioOne (www.bioone.org) is an electronic aggregator of bioscience research content, and the online home to over 160 journals and books published by not-for-profit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. RHODORA, Vol. 112, No. 949, pp. 1–21, 2010 E Copyright 2010 by the New England Botanical Club EVIDENCE THAT SOIL ALUMINUM ENFORCES SITE FIDELITY OF SOUTHERN NEW ENGLAND FOREST TREES 1,2 1 SETH W. BIGELOW AND CHARLES D. CANHAM 1Cary Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545 2USDA-FS, Pacific Southwest Research Station, 1731 Research Park Drive, Davis, CA 95618 e-mail: [email protected] ABSTRACT. Tree species composition of hardwood forests of the northeastern United States corresponds with soil chemistry, and differential performance along soil calcium (Ca) gradients has been proposed as a mechanism for enforcing this fidelity of species to site. We conducted studies in a southern New England forest to test if surface-soil Ca is more important than other factors in determining survival of seedlings of six common canopy tree species. Our hypothesis was that the calcicole species Acer saccharum and Fraxinus americana would show elevated survival rates at higher Ca levels, and that the calcifuge species A. rubrum, Fagus grandifolia, Quercus rubra, and Tsuga canadensis would show lower survival at high Ca. Other factors examined were 1) exchangeable magnesium (Mg), potassium (K), and aluminum (Al); 2) understory light availability; and 3) identity of overstory tree species. In one study, seedlings were transplanted into plots fertilized with Ca or Mg sulfate and survival was measured over 2 years. In the other study, 1-year or 2-year survival of naturally established seedlings in stands dominated by mature trees of one of the six study species was followed. Fertilization with Ca or Mg did not affect survival of planted seedlings, but ambient exchangeable Al was strongly negatively correlated with survival of F. americana. Of the three species with sufficient naturally established seedlings, exchangeable Al plus a proxy for light (overstory tree identity) were the most important determinants of survival. Survival of A. saccharum declined and A. rubrum and F. grandifolia increased at higher levels of exchangeable Al. This pattern is consistent with the positions of these species along the soil gradient. We conclude that soil chemistry effects on seedling survival play a role in establishing the soil relationships characteristic of these species as adults, but that Al is more important than Ca in establishing these effects during the seedling stage. Key Words: northern hardwood forest, calcicole, calcifuge, seedling survival, canopy tree, exchangeable calcium, exchangeable aluminum, site fidelity, plant-soil relationship Tree communities in northern hardwood forests of the north- eastern United States are characterized by affinities of tree species for soil characteristics. Calcicole species such as Acer saccharum and Fraxinus americana are most common on soils high in 1 2 Rhodora [Vol. 112 exchangeable base-forming cations such as calcium (Ca) and magnesium (Mg), and the calcifuge species A. rubrum, Quercus rubra, Tsuga canadensis, and Fagus grandifolia are more prevalent in poorer soils that are acidic and high in exchangeable aluminum (Balter and Loeb 1983; Finzi et al. 1998; Pearson 1962; Schwarz et al. 2003; Smith and Vankat 1991; van Breemen et al. 1997). Differential species performance along soil gradients during the regeneration phase is often assumed to be a cause of this pattern (Grubb 1977). Recent attention has centered on soil Ca regulation of forest composition and function in the northeastern United States and elsewhere (Bellemare et al. 2005; Decker and Boerner 1997; Kobe et al. 2002; McLaughlin and Wimmer 1999; Yanai et al. 2005). Augmentation of depleted soils with Ca enhanced seedling survival of Acer saccharum on granite-derived soils of central New Hampshire (Juice et al. 2006), but another study at the same location found a more direct role of aluminum (Al) in determining A. saccharum seedling survival (Kobe et al. 2002). Calcium and Al can interact closely to determine plant performance (Cronan and Grigal 1995); the calcium displacement hypothesis holds that a decreased ratio of Ca to Al (e.g., due to Ca-poor parent material) leads to increased Al saturation of root surface adsorption sites, causing decreased root survival and growth (Bengtsson 1992). Both Ca and Al may affect establishment of mycorrhizal associations which enhance plant nutrition (Coughlan et al. 2000; Kelly et al. 2005). Regardless of the mechanism of Ca effects on seedling survival, questions remain as to whether Ca promotes seedling survival of other calcicole species besides A. saccharum, such as Fraxinus americana, and whether Ca depresses survival of calcifuge species such as Fagus grandifolia. We carried out two studies to evaluate the role of soil effects on seedling survival in establishing plant-soil relationships in a transitional oak–northern hardwood forest in northwestern Con- necticut. Past studies at this site have documented a strong relationship between soil Ca and Mg and overstory tree species composition (Finzi et al. 1998; van Breemen et al. 1997), and this relationship appears to be already established by the sapling phase (Bigelow and Canham 2002). We carried out one experiment by planting seedlings into plots previously fertilized with Ca or Mg, and another by examining survival of naturally established seedlings on substrates that varied in soil chemistry due to differing 2010] Bigelow and Canham—Soil Al and Seedling Survival 3 parent material and local tree species composition (Dijkstra 2003; Dijkstra et al. 2003). We predicted that survival of Acer saccharum and Fraxinus americana seedlings would be greatest on high-Ca and high-Mg soils, and that the reverse would be true for A. rubrum, Fagus grandifolia, Quercus rubra, and Tsuga canadensis seedlings. We measured additional variables that might either contribute to the formation of plant-soil relationships, or influence survival and thus create experimental noise. The additional variables were soil pH and exchangeable Al and K, understory light availability, and identity of overstory species. The rationale for considering light is that shading by overstory plants can severely limit photosynthesis and, by extension, survival (Kobe et al. 1995; Messier et al. 1999). Potassium availability declines with increasing soil acidity at our study site, and potassium deficiency has been implicated in sugar maple decline (Fyles et al. 1994). Measurement of these additional variables, particularly exchangeable aluminum, turned out to be crucial in understanding patterns of seedling mortality at our study site. MATERIALS AND METHODS The study took place on the Canaan Mountain plateau in Litchfield County, Connecticut (41u589N, 73u159W); the mountain is part of the Berkshires range of the Appalachians, and the forest region is transitional between maple and beech-dominated forests to the north and the oak-dominated forests of the central Atlantic region (Nichols 1935). Most of the study took place within the privately owned Great Mountain Forest (GMF). The bedrock of the plateau is mica-schist, resulting in acid soils of low fertility, but there are areas of higher fertility within the forest which may have resulted from glacial deposition of dolomitic limestone scoured from surrounding valleys. Upland soils where the tree species we studied are most prevalent are loamy, well-drained Inceptisols (Dystrudepts and Typic Eutrudepts; Gonick et al. 1970), but Acer rubrum was most prevalent on aquic soils (Aeric Epiquepts, Humic Endoaquepts) that were high in organic matter. Exchangeable calcium in surface soils at our study sites varies 21 from .200 mmolc kg —suitable for agriculture—to below detection limits, but most areas are low in calcium (Table 1: reanalysis of data in Bigelow and Canham 2002). Previous work at GMF has shown considerable variation in surface-soil cations at 4 Rhodora [Vol. 112 Table 1. Variation in exchangeable nutrients and soil pH throughout the study area. Upper 0.15 m of soil. Sample size is 461 except for pH (N 5 1070). 21 Units are mmolc kg except for pH. Data are from Bigelow and Canham (2002). Soil Nutrients Mean SD Max Ca2+ 42.6 47.6 361 Al3+ 31.0 29.3 180 Mg2+ 12.0 13.4 96 K+ 3.0 2.4 26 pH 4.7 0.64 7.2 the scale of influence of individual trees (Dijkstra 2003; Dijkstra and Fitzhugh 2003; Dijkstra et al. 2001; Dijkstra et al. 2003; Finzi et al. 1998; van Breemen et al. 1997). The pH of surface soils is significantly correlated with concentrations of major cations: Al3+, + + K , and NH4 are predominant on more acid soils (e.g., pH , 4.5), and Ca2+ and Mg2+ are predominant at higher pH. Fertilization study. This study was carried out by planting seedlings into fertilized plots that had previously been established for an experiment on growth limitation of saplings (Bigelow and Canham 2007). Two fertilizer treatments in addition to the control 21 21 were used: 320 kg ha yr of calcium as CaSO4, and 21 21 49 kg ha yr of magnesium as MgSO4, continued for three years (1999–2001).